It has long been known that the heart muscle provides its pumping function in response to electrical events which occur in the atrium and ventricle of the heart. Conductive tissue connects the atrium and the ventricle and provides a path for the conduction of electrical signals between the two areas. In a normal heart, a natural atrial event spontaneously occurs in the atrium and a corresponding ventricular event occurs later in the ventricle after a time interval typically called the A-V interval. After the ventricular event a new atrial event occurs in the atrium to trigger a succeeding ventricular event. The synchronized electrical events occurring naturally in the atrium and ventricle cause the heart muscle to rhythmically expand and contract and thereby pump blood throughout the body.
In a diseased heart atrial and ventricular events may not naturally occur in the required synchronized manner and the pumping action of the heart is therefore irregular and ineffective to provide the required circulation of blood within the body. The required synchronized activity of such diseased hearts can be maintained by a pacemaker which applies synchronized stimulating pulse to either or both the atrium and the ventricle to pace the heart.
In the early stages of pacemaker development, pacemakers were employed to asynchronously stimulate the ventricle of the heart without regard to natural electrical activity occurring in either the atrium or the ventricle. Although this approach had the advantage of simplicity, there was considerable risk due to the fact that paced ventricular events could interact with natural ventricular events to cause the heart to go into a dangerous arrhythmia.
As the art of pacing advanced, pacemakers were provided with circuitry which sensed the occurrence of natural ventricular and atrial activity and paced the heart in either the atrium or ventricle only when required to maintain proper operation of the heart. A dual chamber pacemaker can operate in what is known as DDD mode, wherein electrical events are sensed in the atrium and ventricle and the atrium and ventricle are paced accordingly. Pacemakers may also be operated in VDD mode to sense electrical events in the atrium and ventricle but pace only in the ventricle. Other pacemaker modes of operation are employed to sense or pace in either the atrium or the ventricle, as required for the particular needs of a patient.
It has been found that pacemakers which operate in the DDD or VDD modes can, under certain circumstances, sustain a dangerous tachycardia condition. A pacemaker sustained tachycardia condition is defined as an operational pacing state wherein the pacemaker erroneously stimulates the ventricle of a heart at a dangerously high rate for sustained periods of time.
Pacemaker sustained tachycardia is initiated when a ventricular event occurs at a time during which the connective tissue between the atrium and ventricle can transmit retrograde electrical signals from the ventricle to the atrium. The conduction of the ventricular signal to the atrium may cause an atrial depolarization. The pacemaker senses this retrograde atrial signal and then paces the ventricle at an A-V time period following the signal. The paced ventricular signal is conducted to the atrium where it again causes an atrial depolarization. The pacemaker therefore continues to pace the ventricle at a relatively high rate defined by the sum of the A-V interval and the retrograde conduction time between the ventricle and atrium. This high rate may be sustained indefinitely by the pacemaker, because the retrograde condition ensures that the pacemaker detects what appear to be high rate atrial events and tracks these events by generating corresponding high rate ventricular paces. This pacemaker sustained tachycardia condition overstimulates the heart, at considerable danger to the patient.
One way of minimizing the impact of pacemaker sustained tachycardia is to provide a ventricular tracking limit for a maximum ventricular pacing rate. A pacemaker will not stimulate the ventricle faster than the maximum pacing rate. If such a limit is imposed, however, there can be a progressive loss of A-V synchrony or coordination between the atrial and ventricular chambers of the heart. For example, if the atrial rate is slightly faster than the ventricular tracking limit, then the sensed atrial event would be detected by the pacemaker earlier and earlier in each cardiac cycle. The time between ventricular pacing pulses would be constant, but the delay between a sensed atrial event and a ventricular pace would appear to lengthen in each cycle, since the maximum ventricular pacing rate would be the controlling parameter.
In a DDD pacer, sense amplifiers in the atrium and the ventricle are usually disabled during a pacing pulse, and for a selected period of time thereafter. This is called a pacemaker refractory period. For example, if a atrial event occurs during an atrial pacemaker refractory period, it will not be detected by a most pacemakers. When the pacer is stimulating the ventricle at its ventricular maximum pacing rate, if there is a sustained atrial tachycardia, the delay between the detected atrial event and the ventricular pace will become longer and longer until the atrial event falls within the atrial pacemaker refractory period caused by the ventricular pace of the preceding cycle. When the atrial event falls within the atrial pacemaker refractory period, it is not detected by the ordinary pacemaker. This causes the pacemaker to skip a ventricular beat after which atrial/ventricular synchrony is usually restored with the next properly sensed atrial event. This pause is referred to as Wenckebach behavior.
In DDD pacing, however, the pacemaker is programmed to stimulate the atrium, as well as the ventricle. If a atrial event is not detected, the pacemaker may improperly pace the atrium during the Wenckebach pause. Such an event could produce conditions to permit retrograde conduction to the atrium after the ventricular pacing pulse. Even if the atrial pacing pulse caused a contraction in the atrium, it would effectively accelerate the atrial rate, which is probably already faster than the ventricular tracking limit.
This atrial pace during the Wenckebach pause can sometimes be avoided by lengthening the V-A delay during the pause to give more time for the pacemaker to sense the events in the atrium. Whenever the ventricle is paced at the maximum pacing rate, the ventricle to atrium (V-A) delay, is lengthened by a predetermined value. This procedure is utilized by the assignee of the present invention in one of its marketed products, the Cosmos.RTM. Pacemaker. This method is effective, but it has certain limitations. For example, if a premature atrial contraction occurs, it may cause a ventricular pace which is momentarily at the ventricular tracking limit. If instantaneous ventricular rate is the only criteria for lengthening the V-A delay period, the premature atrial contraction will cause an improper lengthening of the VA delay. In addition, sensing of atrial noise may cause the same response.
In dual chamber pacermakers, atrial paced events can be erronously sensed in the ventricle as well. This condition is known as "cross-talk", where events in one chamber interfere with the operation of the other chamber. If an atrial paced event is erroneously detected in the ventricle as a sensed event, the ventricular output of the pacemaker would be inhibited. The ventricle would not be paced by the pacemaker, and the patient's cardiac output could decline. To avoid this situation, pacemakers turn off or "blank" the sense amplifier in the ventricle during an atrial pulse and for a short time thereafter. This ventricular blanking period is a time when the condition of the ventricle will not be sensed by the pacemaker. The ventricular blanking period should be kept as short as possible so that all valid ventricular events will be sensed.
The duration of the ventricular blanking period has, in the past, been selected with reference to the energy level of the atrial output pulse, as set by the attending physician. More advanced pacemakers, however, are capable of dynamically adjusting both the energy level of the atrial output pulse and the sensitivity of the ventricular sense amplifier. Variations in both the atrial output pulse energy level and the ventricular sense amplifiers sensitivity affect the duration of the optimum ventricular blanking period.